Turkcell Overcharges for Internet Service

Turkcell is a company that should not be trusted. They advertised a home portable internet plan with 250 GB for ₺635. When I signed up, my first bill was ₺700. When I inquired about this, they said it was mentioned in the contract. However, that was not communicated to me. They insisted that their statements were important, and since I signed, I had to accept it.
I wanted to cancel my service, and despite having a supposed 3-month trial period, they charged me a cancellation fee of ₺800. For the past five months, my bills have never been consistent; they have always been about ₺90 higher than the previous month. I purchased additional packages twice for my classes, but the app is subpar and often fails to process transactions. I couldn't cancel the service through the app due to maintenance issues.
After providing so much information to customer service, I was told that my additional package would no longer renew. The representative claimed to have canceled it, but my internet service expired yesterday, and they charged me ₺200 for an additional package. I told them I had canceled it, but they asked which day I called, saying it wasn't recorded in the system. They refuse to accept my last call date. I am convinced they are mocking me.
Despite this, I still want to cancel my service. Is a cancellation fee of ₺1,800 reasonable? I keep pointing out that my bills have increased, but I receive no response, and they continue with their processes. The robotic responses are frustrating. My advice to others is to stay without internet rather than make a deal with Turkcell. They have a contract, but they do not stand behind their own words.